Light valves have been known for over seventy years for the modulation of light and are thus well known in the art. As used herein, a “light valve” is defined as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent electrically conductive coatings. The cell contains a light-modulating element, which may be either a liquid suspension of particles or preferably a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as “a liquid light valve suspension” or simply a “light valve suspension”) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement. Hence, a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus usually relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state. Light valves of the type described herein are also known as “suspended particle devices,” or “SPDs.”
Light valves have been proposed for use in numerous applications including, without limitation, e.g., alpha-numeric and television displays, windows, sun-roofs, sun-visors, filters, toys, apparel, mirrors, eyeglasses and the like to control the amount of light passing therethrough or reflected therefrom, as the case may be.
For many applications, as would be well understood in the art, it is preferable for the material which is to be activated, i.e., the light modulating element, or “the activatable material”, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film, in which droplets of liquid suspension are distributed, is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of liquid suspension can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage. The term, “light valve film” as used herein refers to a film having droplets of a liquid suspension of particles distributed in the film.
U.S. Pat. No. 5,409,734 exemplifies a type of light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking emulsions are also known. See U.S. Pat. Nos. 5,463,491 and 5,463,492, both of which are assigned to the assignee of the present invention and incorporated by reference into this specification.
The following is a brief description, for purposes of illustrating the invention, of the components of a representative light valve suspension.
1. Liquid Suspending Media and Stabilizers
A liquid light valve suspension for use in the present invention may be any liquid light valve suspension known in the art and may be formulated according to techniques well known to one skilled in the art. The term “liquid light valve suspension” means, as noted above, a “liquid suspending medium” in which a plurality of small particles are dispersed. The “liquid suspending medium” comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
The liquid light valve suspensions useful in the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful in the invention, include, but are not limited to, the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565, which are also incorporated by reference herein. In general, one or both of the liquid suspending medium or the polymeric stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles but which also dissolves in one or more of the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, two or more solid polymeric stabilizers may serve as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles. The coated particles are thereafter re-coated with one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and which also dissolves in the liquid suspending medium to provide dispersion and steric protection for the particles. Liquid polymeric stabilizers may also be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492 mentioned above.
The polymeric stabilizers described in the three paragraphs above are confined to the liquid suspensions and serve to reduce the tendency of the particles to agglomerate and to keep the particles dispersed and in suspension. These polymeric stabilizers should not be confused with the emulsifiers of the present invention, however, which are polymeric stabilizers of a different kind having an entirely different purpose, as disclosed and discussed hereinafter.
2. Particles
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light-absorbing or light-reflecting.
Although many types of particles can be used with the present invention, because of the great body of prior art knowledge relating to use of polyhalide particles in SPD liquid suspensions, SPD light valves have usually employed anisometric polyhalide particles of colloidal size. The term “colloidal” as used herein means that the particles generally have a largest dimension averaging about 1 micron or less. Preferably, the largest dimension of most of the particles used in a light valve suspension should be less than one-half of the wavelength of blue light, i.e., 2000 Angstroms or less, to keep light scatter extremely low. As used herein, the term “anisometric,” which refers to particle shape, means that at least one dimension of a particle is larger than another dimension, e.g., the particle length is greater than its width or its diameter as the case may be.
As mentioned above, it is preferable to use a plastic film as the activatable material, i.e., the light modulating unit of an SPD light valve. In the preferred type of SPD film, very small size droplets of liquid suspension are distributed throughout a cross-linked polymer matrix. Such a film is produced from an emulsion comprising (1) a cross-linkable liquid matrix polymer; (2) droplets of liquid suspension which are immiscible with the cross-linkable liquid matrix polymer; and (3) an effective amount of a photoinitiator. Cross-linking (i.e., curing) the emulsion can be accomplished by a variety of means depending on the matrix polymer, e.g., by subjecting the emulsion to heat, or to electron beam radiation or ultraviolet radiation. If heat curing is employed, a catalyst would be used instead of a photoinitiator. In order to avoid confusion and enhance understanding of the present invention, the invention is discussed hereinafter only in terms of cross-linking the emulsion by subjecting it to ultraviolet radiation, with the understanding that, as indicated above a variety of additional cross-linking techniques are available for use.
Advanced types of SPD emulsions and SPD films and methods of making them are disclosed in U.S. Pat. No. 6,900,923 B2 (Chakrapani et al.), which is incorporated by reference in this specification in its entirety. The Chakrapani et al. patent discloses SPD films made from an emulsion comprising (1) a matrix polymer comprising poly(diphenyl dimethyl siloxane) and a cross-linkable monomer such as 3-acryloxypropylmethyl dimethoxy silane; (2) a liquid suspension comprised of polyiodide particles coated with nitrocellulose polymer and dispersed in a polymeric liquid such as poly(lauryl methacrylate); and (3) a photoinitiator such as IRGACURE® 819 (bis(2,4,6-trimethylbenzoyl)-phenyphosphineoxide) (available from CIBA Specialty Chemicals). When the matrix is cross-linked by subjecting a layer of the emulsion to ultraviolet radiation, an SPD film is formed. Certain other non-polymeric liquids disclosed in the Chakrapani et al. patent may optionally be used as part of the liquid suspension. Methods of producing colloidal polyiodide particles coated with nitrocellulose polymer, which can be dispersed in any suitable liquid suspending medium, such as but not limited to a liquid ester such as poly(lauryl methacrylate), are well known in the prior art. Without limitation thereto, see U.S. Pat. No. 5,516,463 entitled “Method of Making Light-Polarizing Particles” assigned to the assignee of the present invention and incorporated herein by reference.
Heretofore, it has not been found necessary to employ a separate emulsifier in order to make a reasonably stable SPD emulsion provided that one cured the well-mixed emulsion quickly after coating the emulsion on a substrate. However, for reasons disclosed herein, an emulsifier, which is a material, preferably a liquid (but sometimes a gel or a solid), can be advantageously used in an SPD emulsion. A first part or segment of the emulsifier is usually soluble in one phase of an emulsion but insoluble in the second phase of the emulsion, and a second part or segment of the emulsifier usually has the opposite solubility characteristics of said first part of said material. Because of these solubility properties, an emulsifier helps to make the two immiscible phases of an emulsion more compatible with one another, which has the effect of stabilizing the emulsion. As is disclosed in more detail hereinafter, however, a material may serve satisfactorily as an emulsifier even if its solubility is somewhat different than described in this paragraph, provided that its segments have appropriate characteristics in accordance with those of the emulsifiers of the invention.
The inventors have observed that if one does not coat and cure a well mixed relatively low viscosity emulsion quickly enough, there is a tendency for the droplets of an uncured SPD emulsion over time to coalesce so as to form larger diameter droplets. Although coalescence of droplets can occur at higher viscosities, such coalescence occurs more rapidly if the viscosity of the emulsion is relatively low, e.g., 5,000 centipoises or less at 25° C. The viscosity of an emulsion can be affected by raising or lowering the molecular weight of one or more of the polymers used in the emulsion. Moreover, for the purpose of coating film in mass production it may sometimes be desirable to lower the viscosity of the uncured emulsion being coated on a substrate on a moving web, for example, by heating the emulsion, which would lower its viscosity and thereby speed up the process of droplet coalescence. Also, the process of mass producing SPD film on a moving web may itself consume a significant amount of time which could give droplets more time to grow by coalescing before curing occurs. Rapid coalescence of droplets occurs if the second transparent electrically conductive-coated substrate is applied to the uncured emulsion coated on the opposing transparent conductive-coated substrate prior to being cured.
The inventors have also noticed that, in general, otherwise identical SPD films comprising relatively small average sized droplets, e.g., 1-2.5 micron diameter droplets, have superior performance properties compared to SPD films comprising larger average sized droplets, e.g., 12.5 microns in diameter or greater, when activated with the same voltage and frequency. Specifically, films comprising the relatively small average sized droplets exhibit a significantly lower (darker) off-state transmission, greater light transmission range and lower on-state haze than the films comprising relatively larger average sized droplets.
Accordingly, it would be highly desirable to have an emulsifier which would retard the coalescence of the droplets, even at elevated temperatures.
In U.S. Pat. No. 5,463,492, assigned to the assignee of the present invention, an SPD liquid matrix polymer and its method of synthesis is disclosed, wherein the polymer comprises (1) a main chain that is insoluble in the liquid suspending medium of the droplets and (2) pendant polymeric groups that are soluble in the liquid suspending medium. Because of its structure, the liquid matrix polymer is referred to in Column 10 of U.S. Pat. No. 5,463,492 as a “cross-linkable copolymer emulsifier.” Lines 11-12 of Column 10 state, as one of its claimed advantages, “Thus, the cross-linkable copolymer emulsifier does not require the use of a separate emulsifier.”
It has been found by the inventors, however, that the cross-linkable copolymer emulsifier also has several serious disadvantages. It is very difficult and expensive to synthesize or obtain. Also its shelf life is poor. If the copolymer degrades it may become unusable. Moreover, it limits one's flexibility in formulating how much emulsifier to use, a problem that can be avoided if instead, a separate non-crosslinkable emulsifier were used to stabilize an emulsion.
There has thus been a long felt need for a suitable separate emulsifier capable of stabilizing emulsions of the type described in U.S. Pat. No. 6,900,923 B2 mentioned above. This need is well met by the emulsifiers according to the present invention without incurring the attendant disadvantages of the prior art emulsifiers discussed above.